docs: def ssh_directory_for_unit(application_name, user=None):
      """Return the directory used to store ssh assets for the application.
  
      :param application_name: Name of application eg nova-compute-something
      :type application_name: str
      :param user: The user that the ssh asserts are for.
      :type user: str
      :returns: Fully qualified directory path.
      :rtype: str
      """
      if user:
          application_name = "{}_{}".format(application_name, user)
      _dir = os.path.join(NOVA_SSH_DIR, application_name)
      for d in [NOVA_SSH_DIR, _dir]:
          if not os.path.isdir(d):
              os.mkdir(d)
      for f in ['authorized_keys', 'known_hosts']:
          f = os.path.join(_dir, f)
          if not os.path.isfile(f):
              open(f, 'w').close()
      return _dir